On 04/18/2018 06:53 PM, Andy Lutomirski wrote:
On Tue, Apr 17, 2018 at 8:00 AM, Denys Vlasenko <dvlas...@redhat.com> wrote:
This means that the new behavior is there for some 8 years already.
Whoever was impacted by it, probably already switched to the new ABI.
Current ABI is "weaker", it allows kernel to save fewer registers.
Which is generally a good thing, since saving/restoring things cost
cycles, and sometimes painful on entry paths where you may desperately
need a scratch register or two. (Recall this one? -
...
movq %rsp, PER_CPU_VAR(rsp_scratch)
movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
/* Construct struct pt_regs on stack */
pushq $__USER_DS /* pt_regs->ss */
pushq PER_CPU_VAR(rsp_scratch) /* pt_regs->sp */
...
wouldn't it be _great_ if one of GPRs would be available here
to hold userspace %rsp?
)
But this is the int $0x80 entry, which uses the stack sanely and
doesn't have this problem at all.
It was a general point why not committing to save every register
may help on the callee (in this case kernel) side.
